A6 C5, 2004, Intermittent loss of boost power

[bandittman]

Hi,
Been having a problem for about two weeks. Drove car harshly for about a mile as I was late for an appointment; suddenly lost boost power. Thought I'd blown the turbo. Took car to local mechanic who read the codes. He confirmed turbo was OK. Codes reported were:-

P1524 Fuel pump relay : Open circuit/short circuit to ground Mechanical Failure
P1549 Wastegate bypass regulator valve : Open circuit/short circuit to ground Mechanical Failure
P1619 Glow plug relay : Open circuit/short circuit to ground Mechanical Failure
P1659 Fan 1 Control circuit : Short circuit to B+ Mechanical Failure
P3005 High heat output relay : Short circuit to B+ Mechanical Failure

Mechanic commented that they didn't make sense as there did not appear to be any 'mechanical' failures in the car. He cleared the codes.

To cut a long story short, the problem has kept recurring. The codes have been read (7 times) and cleared each time. The history up to yesterday is:-

N.B. The codes read are given in the column 'Read x'; in Reading 3, I had someone else read the codes and I didn't capture them apart from the N75 code.

Read 1
P1524 Fuel pump relay : Open circuit/short circuit to ground
P1549 Wastegate bypass regulator valve : Open circuit/short circuit to ground
P1619 Glow plug relay : Open circuit/short circuit to ground
P1659 Fan 1 Control circuit : Short circuit to B+
P3005 High heat output relay : Short circuit to B+

Read 2
P1524 Fuel pump relay : Open circuit/short circuit to ground
P1619 Glow plug relay : Open circuit/short circuit to ground
P3005 High heat output relay : Short circuit to B+

Read 3
Different diagnostic tool; didn't capture codes but there was an N75 fault code present
P1546 Boost Pressure Regulation Valve (N75) : Short to Plus

Read 4
P1524 Fuel pump relay : Open circuit/short circuit to ground
P1619 Glow plug relay : Open circuit/short circuit to ground
P1659 Fan 1 Control circuit : Short circuit to B+
P3005 High heat output relay : Short circuit to B+

Read 5
P1524 Fuel pump relay : Open circuit/short circuit to ground
P1619 Glow plug relay : Open circuit/short circuit to ground
P3005 High heat output relay : Short circuit to B+

Changed Glow Plug Relay for 2nd hand part off fleabay - no change.

Read 5
P1524 Fuel pump relay : Open circuit/short circuit to ground
P1619 Glow plug relay : Open circuit/short circuit to ground
P3003 Relay for Low-output Preheating of Coolant (J359) : Short to Plus
P3005 High heat output relay : Short circuit to B+

Read 6
P1524 Fuel pump relay : Open circuit/short circuit to ground
P1619 Glow plug relay : Open circuit/short circuit to ground
P1546 Boost Pressure Regulation Valve (N75) : Short to Plus
P3005 High heat output relay : Short circuit to B+

Read 7
P1524 Fuel pump relay : Open circuit/short circuit to ground
P1619 Glow plug relay : Open circuit/short circuit to ground
P3005 High heat output relay : Short circuit to B+





So probably the three codes P1524, P1619 and P3005 have occurred every time, and the remainder just occasionally.

The other points to note are:-
1) just before all this happened I changed the Coolant Temperature Sensor; I've since checked and there was no water ingress to the switch/connections.
2) After Read 2, the Hazard Warning Switch module (also controls the indicators) went up in smoke (actually one one board chip burnt out) - coincidence? I replaced it and has not been a problem since.
3) I changed the Glow Plug Relay between Reading 4 and 5, but this has not solved the problem (not unsurprisingly).
4) Sometimes the boost is OK when I start off, but then fails after driving for 5, 10, 30 minutes - seems random.

I was going to replace the N75 valve at weekend, but I think the codes appearing for this may be a red herring.

It could be an electrical fault, maybe something shorting temporarily (?), but I have no idea where to start. I think it's difficult to get to the ECU to look for corrosion as the box has two rear bolts hidden by some plastic grill overhang over the wipers etc.

I have read many similar threads on the forum, but none seem exactly the same as this problem; Any pointers as to where to start would be gratefully received. Thank you.

[amiram]

Perhaps an electrical harness is not well connected or damaged, One that is related to all the faults. Fuel pump relay, Glow plug relay, High heat output relay ,N75 valve ... Or a damaged electrical wires cable

[bandittman]

Yes, it could be a bad connection. I will carry on looking.

UPDATE - drove for 2 miles yesterday; lost boost and engine warning light came on. I had the Ross-Tech VCDS-lite with me so I read the codes. The 'usual three were present P1524, P1619 and P3005. Without turningthe engine off, I cleared the codes and almost immediately, the "P1546 Boost Pressure Regulation Valve (N75) : Short to Plus" appeared. I cleared this code, but it came straight back on. I switched engine off, cleared all codes, switched engine on, and there were no codes present and none came on!

Does that behavior seem normal? Does it mean that the real fault is the N75 valve? Or am I no further on in solving this problem!? Any thoughts welcomed, thanks.

[Vagnuts]

sound like you may have the dreaded sticking turbo vanes/takes to long to make boost(Mister Muscle in exhaust side of turbo while working the actuator-will clear it if thats what it is !)--,putting it into limp mode, or bad earth-which I fairly sure that all will be running to the same point to earth !!

[bandittman]

Hopefully it's just a bad earth. But I'm guessing it will take some finding! Thanks for your suggestions (inc Mr Muscle - seen this on youtube too).

[Vagnuts]

we have a 2.5 V6 TDI A6 C5 03plate--turbo very easy to get to--I didnt catch what engine u got- but-the 4pot is easy to get to as well, EGR pipe off, then feed tube into exhaust side an fill wi MM, but because there so Many short to ground's---I do think it be an earthing prob, start in engine bay an work out from there !! Ps the MM works--I have used it plenty times just last week my own A 3 8L PD 130 needed the MM treatment--worked spot on !!!

[bandittman]

Thanks for the suggestions so far; I think it maybe an electrical problem, so I'm not going down the Mr Muscle route yet. Problems is intermittent; there most of the time but sometimes goes away for a short while. Seems random.

Today I managed to reach the three N75-type valves present and measured their resistances; can anyone tell me if these are in range or out of spec?

1) the 1K0 906 283 A measured 33 ohms - I think this is known as a solenoid valve

2) the 8E0 906 628 A or B (not sure if A or B) measured 16 ohms - I think this is known as a pressure converter

3) the 8E0 906 227 C measured 15.5 ohms - I think this is known as a pressure converter.

Are these correct?